1. Field of the Invention
This invention relates to a thermal head and to a method for the manufacture thereof, and more particularly to a heat-generating heat resistor layer used in the thermal head.
2. Description of the Prior Art
Generally the thermal head has a heat-generating resistor layer for forming heat-generating dots, a power feeding conductor layer for feeding electricity to the heat-generating resistor layer, and a protective layer for protection against oxidation and wear superposed sequentially on an electrically insulating substrate such as a ceramic sheet coated with a thin glass layer.
Heretofore, a film of tnatalum nitride has been used as the heat-generating resistor layer mentioned above. The tantalum nitride film has the advantage that it excels in thermal resistance, possesses a small temperature coefficient, and adheres tightly with an underlying glass layer.
The film of tantalum nitride is formed by spattering a tantaoum target in a mixed gas consisting of argon gas and a minute amount of nitrogen gas. Even when the nitrogen gas content of the mixed gas is varied more or less during the course of this production, the specific resistance and the temperature coefficient of the produced tantalum nitride film are affected nominally. The presence of the so-called "plateau region" constitutes a major feature of this film. Thus, the conditions of production are easy to control. It is a film resistor material ideal for mass production.
Then, in the pattern formation, the film can be easily etched with a mixed acid consisting of nitric acid and hydrofluoric acid. It can otherwise be dry etched with the CF.sub.4 gas, for example. This ease of fabrication forms one reason for popular acceptance found by the tantalum nitride film.
When the tantalum nitride film is used as the heat-generating resistor in the thermal head, since it offers no sufficient resistance to thermal oxidation, there has existed an established practice of coating the surface of the tantalum nitride film with an anti-oxidant protective film made of silicon oxide. Since the tantalum nitride film and the silicon oxide film adhere to each other with extremely high intimacy, the coating with the silicon oxide film amply enhances the resistance of the tantalum nitride film to thermal oxidation.
Recent years have witnessed proposal of various heat-generating resistor materials highly stable to withstand thermal oxidation. Some of them entail production conditions difficult to control and others exhibit poor processibility as by etching. Thus, very few of them have been adopted for actual use. The aforementioned advantage in terms of production is believed to form one major cause for the conventional widespread adoption of the tantalum nitride film as the heat-generating resistor material.
The use of the tantalum nitride film as the heat-generating resistor in the thermal head is not entirely free from any shortcoming. The problem encountered in this case will be described below.
The curve A of FIG. 1 represents the results of the step-stress test performed on a thermal head using the tantalum nitride film as a heat-generating resistor as heretofore practiced. This is one type of accelerated test generally used in evaluating the thermal stability of a thermal head. This test is carried out by applying a proper pulse voltage to the heat-generating resistor for a prescribed period while measuring a change in the magnitude of initial voltage, repeating the application of pulse voltage while increasing the magnitude of applied pulse voltage stepwise until the heat-generating resistor burns out, and plotting the ratios of change in the magnitude of resistance at the successive steps.
The conditions of the step-stress test are as follows. In the first step, application of a pulse voltage of 7.0 V for 1 m.second is continued in cycles of 20 m.seconds over a total period of 10 minutes. At the end of the first step, the magnitude of resistance is measured. In the second step, the application of a pulse voltage now raised to 7.5 V is continued under the same conditions as in the first step. In the subsequent steps, the application of a pulse voltage successively raised by a unit increment of 0.5 V per step is continued. This voltage application is repeated until the ratio of change in the magnitude of resistance, .DELTA.R/R, rises past 10%.
The upper horizontal axis of FIG. 1 is a scale for the highest temperature of the surface of the heat generator under varying magnitudes of applied voltage. This temperature is measured with an infrared spot thermometer.
In the thermal head using the conventional heat-generating resistor formed of a tantalum nitride film, the magnitude of resistance of the heat-generating resistor begins to decrease when the applied power is about 22 W/mm.sup.2 and the surface temperature of the heat generator is about 400.degree. C. as indicated by the curve A. Incidentally in the ordinary thermal printer, the surface temperature of the heat generator required for heat generation of the thermosensitive paper is about 350.degree. to 400.degree. C. As the printing speed is increased, the surface temperature of the heat generator is required to be proportionately increased, past 500.degree. C., for example. When the thermal head using the heat-generating resistor of the conventional tantalum nitride is used in printing, the density of the prints produced gradually increases with elapse of printing time. At times, the quality of prints is inferior because of smearing, blurring, etc.
This drawback is ascribable to the aforementioned decrease in the magnitude of resistance of the heat-generating resistor of tantalum nitride at elevated temperatures. Generally, the voltage applied to the thermal head is constant and the magnitude of resistance of the heat-generating resistor of tantalum nitride continues to decrease. Thus, the electric power supplied for heat generation sharply increases relatively and, as the result, the surface temperature of the heat generator is excessively elevated. In this manner, the surface temperature rises past the optimum color-producing temperature of the photosensitive paper to entail the degradation of the quality of print due to smearing, for example. Since the elevated temperature is much higher than the temperature normally required, it burns out the heat-generating resistor and renders it useless.
An object of this invention is to provide a thermal head free from the aforementioned drawbacks suffered by the conventional countertype and capable of producing prints of high quality and a method for the manufacture of this thermal head.